1. Field of the Invention
The present disclosure relates to image sensors using semiconductor substrates, and more particularly, to complementary metal oxide semiconductor (CMOS) image sensors having photodiodes.
2. Description of the Related Art
Image sensors are semiconductor devices that convert optical images into electrical signals. In particular, complementary metal-oxide semiconductor (CMOS) image sensors include photodiodes that receive and store optical signals and form images using control devices that control or process the optical signals. Control devices can be fabricated using CMOS fabrication technology. Therefore, a process of fabricating CMOS image sensors can be realized, and a plurality of signal processing devices can be integrated onto a single chip.
Drawbacks and disadvantages of conventional CMOS image sensors will now be described with reference to FIG. 1. Referring to FIG. 1, a unit pixel of a conventional CMOS image sensor includes a photodiode 185, a transfer transistor 188, a reset transistor 158, a drive transistor 168, and a selection transistor 178.
The photodiode 185 receives optical energy and generates electric charges accordingly. The transfer transistor 188 may control the transmission of the generated electric charges to a floating node 190 using a transfer gate line TG. The reset transistor 158 may control a supply voltage Vdd using a reset gate line RS and reset the potential of the floating node 190. The drive transistor 168 may function as a source follower amplifier. The selection transistor 178 is a switching device that can select a unit pixel using a selection gate line SEL.
The supply voltage Vdd may be output to an output line OUT via the drive transistor 168 and the selection transistor 178. The drive transistor 168 may be controlled by the potential of the floating node 190. When the reset transistor 158 is turned on (that is, when a reset operation is performed), the potential of the floating node 190 is proportional to the supply voltage Vdd and inversely proportional to a voltage drop in the reset transistor 158.
To lower the potential of the floating node 190 during the reset operation, the reset transistor 158 may be formed as a depletion transistor. However, the drive transistor 168 and the selection transistor 178 constituting an output circuit may be formed as enhancement transistors in consideration of off-leakage current.
As the integration density of CMOS image sensors increases, pixel sizes are reduced, and operating voltage becomes lower. Accordingly, voltage that can be output from a pixel becomes lower, and an output signal range of the pixel is reduced as a result.